How can I find out how much disk space the WinSxS folder is really occupying?
If you ask Explorer or some other disk space utility for the size of the WinSxS folder, you’ll get an inflated value because WinSxS consists mostly of hard links into other parts of Windows, so it doesn’t really occupy space on its own, but naïve disk space calculation algorithms don’t take hard links into account (in part because it’s not clear how one should account for them).
The command to analyze the contents of the WinSxS directory is
dism /Online /Cleanup-Image /AnalyzeComponentStore
This Web page explains how to interpret the results.